REGULATION OF DIHYDROPYRIDINE RECEPTOR AND RYANODINE RECEPTOR GENE-EXPRESSION IN REGENERATING SKELETAL-MUSCLE

One of the the major properties of mature skeletal muscle is its abili ty to regenerate after injury. The purpose of the present study was to determine whether the expression of genes encoding the dihydropyridin e receptor calcium channel (DHPR) and the ryanodine receptor (RyR), wh ich play a critical role in excitation-contraction coupling, is regula ted by skeletal muscle regeneration. The process of regeneration was i nduced by bupivacaine injection in surgically exposed rat extensor dig itorum longus (EDL) muscle. After total RNA isolation from the injecte d and the contralateral control EDL muscles performed 3, 7, 15 and 30 days following injection, Northern blot and RNase protection assays we re carried out with four cDNA probes specific for the skeletal and car diac muscle isoforms of both the DHPR al-subunit and the RyR. After 3 days, an initial precipitous decrease in the expression of the genes e ncoding the skeletal muscle isoforms of the DHPR and RyR was observed, followed by an increase. Moreover, regenerating skeletal muscle trans iently expressed mRNA for the DHPR cardiac isoform, mainly at the begi nning of regeneration. No expression of mRNA for the cardiac RyR was o bserved. Contraction experiments, performed using EDL muscle at the sa me times after bupivacaine injection, showed that twitch amplitude was markedly decreased in the absence of external calcium, but only durin g the early stages of regeneration. Similar findings in relation to ex pression of skeletal and cardiac muscle DHPR message were previously r eported from experiments conducted during early developmental stages u sing fetal skeletal muscle and muscle eel cultures [Chaudhari N, Beam KG (1993) Dev Biol 155:507-515]. These results suggest that expression of the DHPR cardiac isoform in skeletal muscle could explain certain cardiac-like aspects of excitation-contraction coupling of regeneratin g skeletal muscle and developing skeletal muscle as well.